A supercharger incorporated in an engine of an automobile or the like is adapted to supply compressed air to the engine to increase an engine power output by rotating an impeller at an exhaust side by an exhaust gas from the engine to rotate another impeller disposed in an intake side coaxially with the impeller at the exhaust side. Since the exhaust side impeller is exposed to the high temperature exhaust gas discharged from the engine, it has been made from a heat resisting Ni-based superalloy by a lost wax casting process because of its less complicated form. On the other hand, since the intake side impeller is not exposed to a high temperature and hence, it is made mainly of an aluminum alloy. In recent years, however, since the impeller is required to rotate under a higher speed in order to improve the combustion efficiency, a titanium alloy has been tried for use because of its light weight and a high strength. Further, a magnesium alloy has been also tried for use because it will be able to realize a much more weight reduction as compared with the titanium alloy.
In many cases, the intake side impeller has a complicated blade form in which a plurality of full and splitter blades of two types different in form from each other are usually arranged alternately and adjacent to one another in order to provide an increase in compression rate of the compressed air. In the case of a cast impeller made of an aluminum alloy, it has been produced by a plaster molding process wherein a plaster casting mold is used, which mold is produced with utilization of an elastic rubber pattern. An impeller made of a magnesium alloy can be also produced by the plaster molding process. Such a rubber pattern is produced by the following process consisting of producing a master pattern of a impeller; producing a casting mold with utilization of the master pattern; and injecting a silicone-based rubber into the casting mold. According to the rubber pattern, it is possible to reproduce a complicated form of the master pattern while having a small problem in dimensional accuracy.
On the other hand, when an active metal such as a titanium alloy is cast by the plaster molding process, a plaster mold and a molten titanium alloy react heavily with each other, so that such casting is impossible. Thus, an impeller of a titanium alloy has been produced from a cast material by five-axis machining. However, since the titanium alloy is hard to machine, such machining is expensive and unsuitable to mass production. Therefore, with regard to the case of the titanium alloy, it has been tried to use the lost wax casting process according to which it is possible to use a ceramic shell such as zirconia and yttria stable to the titanium alloy.
In the lost wax casting process, it is necessary to produce a sacrificial pattern having the same form as a final product of an impeller by a die casting method. For example, according to a publication of US-2002-0187060-A1 (corresponding to JP-A-2003-94148), there is proposed a titanium compressor impeller produced by a lost wax casting process, in which the blade form is redesigned so that die inserts (i.e. slide dies) can be drawn out of blade portions of a sacrificial pattern. (*Note: In the publication, the casting is referred to as an investment casting.) This proposal is excellent in the point that titanium alloy impellers can be produced relatively inexpensively in mass production.